1. Field of the Invention
The present invention relates to steam generators and particularly to such steam generators for use in nuclear power plants.
2. Description of the Prior Art
Steam generators for use in power plants, and particularly for use in nuclear power plants are well known, such as the type of steam generator manufactured by Westinghouse. One such typical prior art steam generator is shown in FIG. 1 of the drawings. These prior art steam generators utilized in nuclear power plants are normally contained in a containment building, such as one made of concrete. Moreover, these prior art steam generators known to applicant are unitary structures comprising a housing having an upper shell and lower shell portion with a moisture separator, including a swirl vane moisture separator, normally being located in the upper shell portion and with a tube bundle normally being located in the lower shell portion. In such prior art steam generators presently employed in nuclear power plants, the steam generator is completely enclosed and is placed in the containment building prior to the concrete being poured. As a result, once the containment building is sealed there is no way to replace this steam generator without breaking or destroying the containment building. Accordingly, if there is a failure in the tube bundle, it has heretofore been necessary to break the containment building in order to repair the steam generator as that is the only manner in which access can be had to various portions of the steam generator. Moreover, failure in the tube bundle of this prior art unitary type of steam generator has required replacement of the entire steam generator in order to allow for such repair, which complete replacement is quite costly in that such a steam generator costs approximately $15,000,000 by 1976 standards.
With the widespread acceptance and use of nuclear power plants, there have been more and more such tube failures in the tube bundles of such steam generators which have required the power plant to be shut down. This, of course, can be extremely costly inasmuch as the steam generators are a vital component in the operation of the nuclear power plant. Moreover, since such nuclear power plants normally include three or four such steam generators in operation, the cost of replacement and/or repair can become quite prohibitive.
Although the use of modular housing in various types of heat exchanges has been well known, such as disclosed in U.S. Pat. Nos. 1,372,010; 2,228,549; 2,241,209; 1,564,446; 1,790,897; 973,610; 514,338 and 784,192, such techniques, to applicant's knowledge, have not been used with respect to steam generators and particularly steam generators for use in nuclear power plants, despite the serious problems encountered with respect to repair and/or replacement of these steam generators in situ.
With respect to the aforementioned failure in the tube bundle, one of its primary causes has been found to be build-up around the tubes as a result of cooling water or secondary water chemicals, such as phosphates, chlorides, etc. This build-up causes a thinning or weakening in the walls of the individual tubes which ultimately results in their failure. In an effort to overcome these problems resulting from build-up around the tubes, supporting grid structures comprising a plurality of separate grids have been used in place of the steel plates, such as used in the typical prior art steam generator of FIG. 1. An example of such structures are disclosed in U.S. Pat. Nos. 4,021,204 and 4,036,461 which disclose grid-type tube supports wherein the individual grids consist of wide sections which are intersected by thin sections to define the grid apertures, similar to the arrangement shown in FIG. 10, except for the presence of flow holes. Nevertheless, build-up may still occur around the tubes, although it is considerably less than in other prior art arrangements.
The accepted manner presently used for detecting any potential weakening or thinning in the walls of the individual tubes consists of eddy current testing in which current is applied from the tube sheet through the tubes to measure the wall thickness or discontinuance. However, at the location where the tube passes the grid, or a drilled hole plate if one is employed instead, it is difficult to get accurate eddy current readings due to considerable interference by the grid. Thus, such eddy current testing has not proven satisfactory in accurately detecting potential or actual failure of the tubes at the critical points where they pass through the individual grids.
These disadvantages of the prior art are overcome by the present invention.